In answer to the OPs question - it depends. Look at the quality of the discs you're going to use. In general, the better the media, the more likely it is to give you better results at the higher speed.
For example, if I take an 8X-rated Taiyo Yuden disc, MID YudenT002, it can burn well at 4X, 8X and even overspeed well to 12X and 16X. Thing is, most writers accelerate toward the end of the disc, so you *may* see PIE spikes toward the end when overspeeding. Should be well within error correction norms for the reader though.
However, if I'm dealing with an 8X Princo, then yes, the lower the better. Longevity will be an issue as well, with this media.